1. Field of the Invention
The present invention relates to an information recording apparatus such as a laser beam printer and, more particularly, to an information recording apparatus which can perform print processing with high image quality without deteriorating smooth print quality even when the apparatus simultaneously adopts both a technique for improving print quality by smoothing the edges of characters and figures which are printed by smoothing bit-map data representing characters and figures, and a technique for saving toner by adjusting the recording density.
2. Related Background Art
In recent years, laser beam printers using an electrophotographic technique are widely used as information recording apparatuses such as output apparatuses for computers, output units for facsimile systems, so-called digital copying machines for printing image data read from image scanners, and the like.
A laser beam printer will be exemplified below.
In a conventional laser beam printer, a technique for smoothing the edge portions of characters and figures is known.
A laser beam printer adopting such a technique performs a print operation at a resolution of, e.g., 300 dots/inch.
In this case, a character or figure is drawn by printing black dots (.circle-solid. marks) and white dots (.smallcircle. marks) at lattice positions of 300 dots/inch, as shown in FIG. 4. FIG. 4 shows a dot pattern of an English letter "a". At the resolution of 300 dots/inch, the interval between adjacent dots is about 85 microns. In general, the human eye can visually recognize an object as small as about 20 microns. Thus, at the above-mentioned resolution (about 85 microns), the edge portion of a character or figure formed by dots looks jaggy, and such a print operation does not always assure high image quality.
In order to solve this problem, the following approaches are available.
As the first approach, a method of simply increasing the resolution (e.g., 1,200 dots/inch) comes into mind. However, in this case, in order to express an identical area, a bit-map memory having a capacity 16 times (=4.times.4) that of a memory for 300 dots/inch is required, resulting in a very expensive apparatus.
As the second approach, a method of equivalently increasing the resolution in the main scanning direction or in both the main scanning and sub-scanning directions by adding only a buffer memory having a small capacity without increasing the capacity of a bit-map memory, and by modifying print data of a pixel of interest with reference to dot data around the pixel of interest to be printed, is known.
The modified pixel size will be described below with reference to FIGS. 9A to 9D in association with the lower limit of segmentation processing of the modified pixel unit size upon modification of a pixel of interest by a smoothing processing circuit. FIG. 9A shows an original pixel before smoothing modification. FIG. 9B shows a case wherein the modified pixel size for smoothing modification is set to be 1/4 of the size of the original pixel, FIG. 9C shows a case wherein the modified pixel size for smoothing modification is set to be 1/8 of the size of the original pixel, and FIG. 9D shows a case wherein the modified pixel size for smoothing modification is set to be 1/16 of the size of the original pixel.
As the modified pixel unit size becomes smaller, finer smoothing processing can be realized. However, the driving frequency of the smoothing processing circuit increases accordingly, and the smoothing processing circuit must be designed using Bi-CMOS logic or ECL logic, resulting in an expensive circuit.
On the other hand, when the modified pixel unit size is set to be large, pixels with the modified pixel unit size are printed while being sufficiently resolved, and a smoothed portion is printed as a "whisker"-like portion rather than a "blurred" portion, as would be preferable. As a result, the desired smoothing effect cannot be sufficiently expected.
The modified pixel unit size which can satisfy the above-mentioned two contradictory conditions largely varies depending on the toner particle size. Commercially available laser beam printers normally use a toner having a toner particle size of 10 to 12 microns (to be referred to as a normal particle size toner hereinafter). On the other hand, in recent years, in order to improve the resolution, printers which use a toner having a toner particle size of 5 to 6 microns (to be referred to as a small particle size toner hereinafter) are developed or commercially available. In addition to the toner particle size, the optimal modified pixel unit size also varies depending on the light-emission response characteristics of a laser due to a difference in laser driving circuit, the conditions of an electrophotography process, and the like.
In general, a smoothing processing circuit is normally designed as an integrated circuit such as a gate array, i.e., a CMOS including 3,000 to 10,000 gates. In this case, it is not advisable to develop another gate array in correspondence with the above-mentioned difference in toner. When processing corresponding to the difference in toner is assembled in a single gate array, not only does the number of gates increase, but also the integrated circuit requires a logic which can respond to high frequency, e.g., a Bi-CMOS logic, ECL logic, or the like, resulting in an increase in cost. Even when the same toner is used, the modified pixel size changes depending on the light-emission rise characteristics and the electrophotography process of a laser. Therefore, it is not practical to pre-program smoothing processing modes corresponding to many conditions.
The second drawback is a problem posed when the smoothing processing circuit is applied to a printer engine which can switch the print resolution. For example, when the print resolution is switched between 240 dpi and 300 dpi or between 300 dpi and 600 dpi in response to a command, it is difficult to optimize the smoothing effects for these two print densities. More specifically, even when an algorithm which can improve the smoothing effect for one print resolution is used, the smoothing effect for the other print resolution cannot always be optimized.
The third drawback is associated with optimization of smoothing processing for a printer engine which has a function of changing the density of an image to be printed to a relatively low or high density by, e.g., changing the high voltage applied to a developer. When the print density is changed, the effect on a smoothed portion also changes, resulting in deterioration of image quality.
The fourth drawback is associated with the influence, on the smoothing effect, of the use environmental conditions such as the temperature, humidity, and the like of a printer engine. When an environmental condition such as the temperature, humidity, and the like changes, the print density of a printed image changes, and the effect on a smoothed portion also changes, resulting in deterioration of image quality.
As a smoothing processing technique for eliminating these drawbacks, the assignee of the present invention has filed U.S. Pat. No. 5,465,157, U.S. application Ser. No. 07/855,083, filed Mar. 20, 1992, U.S. Pat. No. 5,381,522, U.S. application Ser. No. 07/858,075, filed Mar. 26, 1992, refiled as application Ser. No. 08/430,161, and U.S. application Ser. No. 08/012,634, filed Feb. 3, 1993.
The process of a laser beam printer is set to obtain always the best image density, so that the toner consumption amount per unit area is constant. For this reason, even in a test print mode of an original, the printer consumes toner to obtain the best image density. In this case, in general, as long as characters are sufficiently legible, print cost per page is to be reduced by suppressing toner consumption even if the image density is lowered to some extent.
As a method of controlling the image density, a method of controlling the consumption amount of a toner by controlling the application voltage of a developing bias is a known technique. An image density control method of this type suffers the following drawback since the process condition upon recording is changed.
More specifically, when the recording density is lowered by controlling the developing bias voltage, another image quality deterioration occurs in addition to a decrease in density. That is, when the density is lowered by this method, image fogging (an unnecessary toner is uniformly attached to a non-print portion to deteriorate printed image quality), or image scattering (a toner is scattered to a portion around a printed portion to deteriorate printed image quality) often occurs as a result of the decrease in density. Also, it is difficult to stably maintain a low density in correspondence with a change in environmental condition such as a change in temperature, humidity, and the like. The recording density may be lowered by thinning out recording pixels. In this case, image quality considerably deteriorates. Therefore, when the above-mentioned smoothing processing is performed, the smoothing effect changes at a low density, thus losing smoothness of an edge.